Another Russian satellite is set to re-enter Earth's atmosphere tonight, with predictions for the exact time of arrival extending through to 4:04 tomorrow morning. The location of a potential impact for Cosmos 2176, a missile early warning satellite weighing two tonnes, is as yet unknown.

Three major satellites have now fallen out of orbit in the last five months; NASA's Upper Atmospheric Research Satellite (5.9 tonnes), Germany's Roentgen Satellite (2.4 tonnes) and, as of Sunday, Russia's Phobos Grunt probe (13.5 tonnes). Two of the resulting debris showers fell harmlessly over the Pacific Ocean, and there is no record of the third having caused any damage on the ground.

Despite the relative rarity of such instances, there are millions of objects circling the planet every day, including fully functioning satellites, pieces of equipment left behind by past missions and fragments of debris resulting from collisions between these objects.

Since the launch of the first ever artificial satellite, Sputnik 1, on 4 October 1957, over 28,000 orbiting objects of a measurable size have been created, of which around 550 are currently functioning spacecraft. Each year roughly 75 new launches add to this figure, with orbital space becoming increasingly crowded.

As of 5 October 2011, Russia (including the Soviet Union and CIS) had deployed the most satellites since the dawn of the space age, with 1412, edging out the US which has 1154. By contrast, the European Space Agency has only launched 39.

According to the most recently available records from NASA's Orbital Debris Program Office, there are currently roughly 11,000 large objects (those with a diameter of over 10cm) in orbit, with a further 100,000 between 1cm and 10 cm in in diameter, and millions more of smaller sizes.

Items in the lowest orbits can stay in space for as little as a few days, while others, such as the communications satellites which travel 36,000km above ground, will remain in orbit indefinitely.

While we may be concerned more with a satellite falling out of the sky, collisions between objects in orbit are much more frequent and can result in considerable damage to any equipment involved.

In a typical orbital collision the two objects come together at just under 10km per second, but in the case of a head-on impact, the total velocity can be as high as 16km per second. To provide some context, the highest airspeed ever achieved by a manned aircraft is roughly 1km per second.

These high speeds mean that huge amounts of energy are involved, with the impact of a 1cm aluminium sphere at 10km per second equivalent in force to being hit by a medium sized car travelling at 55mph. A collision with such a fragment of debris can render a satellite useless, and great care is taken to track the larger objects in order to manoeuvre satellites out of their path.

Below is a breakdown of the approximate composition of orbital debris: